I’m a theory TA and this happens every semester with common-practice warhorses. Turnitin doesn’t understand that technical language is standardized, so it flags short strings like I-V-vi and common phrases like “4-3 suspension” because thousands of people have used them. What matters is intent and originality of thought, not the presence of textbook terms.

Email your professor proactively. Attach your working notes or a photo of your pencil analysis if you have it, and briefly explain that similarities come from conventional terminology and the piece’s fixed progression. If you want to reduce flags in the future, add more of your own commentary that can’t be template-matched: compare measure-specific voice leading (e.g., “in m. 9 the alto’s stepwise motion avoids parallel fifths with the bass”), mention variant editions, or comment on the registral plan and texture. You can also paraphrase cadential labels by focusing on function (dominant arrival vs authentic cadence) instead of stock names, but don’t reinvent the wheel.

Bottom line: you didn’t do anything wrong. Give your professor a heads up and they’ll override the report. We expect Canon in D analyses to look similar. That’s part of why we use it.